The present invention relates to a method of controlling the heating of an oxygen sensor fitted to the exhaust system of an internal combustion engine for the purpose of controlling air-fuel mixture air/fuel ratio, and to a system for practicing the method. More particularly, the present invention relates to such a method and device for oxygen sensor heating control which synchronize the detection of an intermittent or pulse voltage applied to an electrical heater element of the oxygen sensor with the time interval in which said heater element is energized, thereby controlling more accurately the power to said heater element.
The present patent application has been at least partially prepared from material which has been included in Japanese Patent Application No. Sho 59-90682(1984), which was invented by the same inventors as the present patent application, and the present patent application hereby incorporates the text of that Japanese Patent Application and the claim or claims and the drawings thereof into this specification by reference; a copy is appended to this specification.
It is known to fit an oxygen sensor to the exhaust system of an internal combustion engine. Such an oxygen sensor typically comprises a solid electrolyte or semiconductor, and varies a generated current or resistance in response to the concentration of oxygen in the exhaust gases of the engine. This electrical signal is fed to a control device which controls the amount of fuel provided to the engine in relation to the amount of air sucked thereinto, and is used for controlling the air/fuel ratio of the air-fuel mixture supplied to the engine by a feedback process. Various such forms of control device, which practice various methods of air-fuel mixture ratio control, are per se known.
The output of the sensor element of such an oxygen sensor varies with temperature, and, particularly when the air/fuel ratio is lean and is in the range of 14.5 to 25, in order for the sensor element to accurately measure the oxygen concentration, said sensor element must be maintained at a temperature higher than a certain critical minimum active temperature. This maintenance of the temperature of the sensor element can be done by using a heater, and oxygen sensors with sensor element heaters have already been proposed, along with methods for operation of such heaters; for example in Japanese Patent Application No. 53-78476, which has been published as Japanese Patent Publication No. 54-13396. Further, in Japanese Patent Application No. 53-83120, which has been published as Japanese Patent Publication No. 54-21393, there has been proposed a method and a system for control of the eletrical power supplied to such an oxygen sensor element heater, in which the power is varied as a function of intake manifold pressure, of throttle opening, and of engine revolution speed, so as to ensure that the oxygen sensor element is kept at a temperature no lower than its minimum active temperature.
The sensor element of such an oxygen sensor fitted to an exhaust system is of course heated up by the exhaust gases in the exhaust system, so the effect of a heater for the sensor element must be controlled to take account of the temperature of these exhaust gases. Now, in an internal combustion engine which is controlled by a throttle valve, the exhaust temperature is largely determined by the amount of air-fuel mixture supplied per engine piston stroke and by engine revolution speed, and if the air/fuel ratio of the air-fuel mixture is constant the amount of such mixture supplied is proportional to the rate of intake air flow. Therefore, in the above mentioned patent applications, the above are used as parameters, and the supply of electricity to the sensor element heater is varied depending on the engine load and the engine revolution speed. Thus, the exhaust temperature is considered to depend on the engine intake flow and engine revolution speed, and the values are determined experimentally in advance with reasonable accuracy. This method and system are adequate to keep the temperature of the sensor element of the oxygen sensor reasonably constant regardless of engine operational conditions.
The heater element of such an oxygen sensor fitted to an exhaust system is typically of a pure resistive load type, using the Joule heating phenomenon for producing heating power, and typically such a heater element has a resistance which increases as its temperature increases. This causes fluctuations in the power dissipated in the heater element, with respect to the same voltage supplied thereto. One approach to tackling this problem is to supply a pulsed voltage signal to the heater element, and to alter the duty ratio of this pulsed voltage signal so as to keep the heater power, as measured by the product of the current through said heater element and the voltage across said heater element, at the desired control target value. In this case, however, the pulse nature of the voltage signal should be allowed for.
Further, ideally the actual voltage across the heater element should be measured, but in practice the supply voltage to the control unit therefor may be used. In this case, however, the internal resistance of the vehicle battery, and in the wiring harness of the vehicle and so on, may mean a fluctuation in voltage of some hundreds of millivolts or so, between the times when the heater is on and the times when it is off. This can be a source of error if it is not allowed for. Also, the saturation voltage across the emitter and the collector of a control transistor for the heater element is a non linear function of the collector current, and so to correct for it could possibly involve rather complicated calculation.